TriFibonacci | Topcoder Archive

Problem Statement

A TriFibonacci sequence begins by defining the first three elements A[0], A[1] and A[2]. The remaining elements are calculated using the following recurrence:

A[i] = A[i-1] + A[i-2] + A[i-3]

You are given a int[] A which contains exactly one element that is equal to -1, you must replace this element with a positive number in a way that the sequence becomes a TriFibonacci sequence. Return this number. If no such positive number exists, return -1.

Definition

Class:: TriFibonacci
Method:: complete
Parameters:: int[]
Returns:: int
Method signature:: int complete(int[] A)
(be sure your method is public)

Notes

The constraints for the elements of the input int[] A do not necessarily apply for the replacement to the missing element.

Constraints

A will contain between 4 and 20 elements, inclusive.
Each element of A will be -1 or between 1 and 1000000, inclusive.
Exactly one element of A will be -1.

Examples

```
{1,2,3,-1}
```
```
Returns: 6
```
```
{10, 20, 30, 60, -1 , 200}
```
```
Returns: 110
```
```
{1, 2, 3, 5, -1}
```
```
Returns: -1
```
No replacement can make this sequence TriFibonacci as 5 is not equal to 1+2+3.
```
{1, 1, -1, 2, 3}
```
```
Returns: -1
```
The missing element must be 0 for this sequence to be TriFibonacci. Since this is not a positive integer, return -1.
```
{-1, 7, 8, 1000000}
```
```
Returns: 999985
```

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